Canned motor pumps are widely used to circulate water in heating and plumbing systems. Examples of canned motor pumps are found in U.S. Pat. Nos. 4,990,068 and 5,549,459. Canned motor pumps are lubricated by the fluid being pumped which typically is water. These pumps are commonly referred to as water lubricated pumps. The pumps are typically driven by an electric motor and the rotor of the motor, as well as the pump impeller, are mounted on a common shaft. An inherent feature of canned motor pumps is that all rotating parts are immersed in the fluid being pumped. Because of that they generally do not require any dynamic seals such as packings or mechanical seals. Since the shaft is immersed in the fluid being pumped, it follows that the bearings supporting the shaft are also immersed in the fluid, usually water. It is a common practice to use sleeve bearings, as opposed to ball bearings, in canned motor pumps.
When constructing sleeve bearings, it is desired to achieve a hydrodynamic operating condition, i.e., complete separation of the shaft and the bearing by a fluid film. From a practical point, this is not consistently possible, particularly with small bearings and a lubricant of very low viscosity, such as water. Therefore, what actually occurs is so-called mixed-film lubrication where some contact between shaft and bearing remains and thus some rubbing is constantly present. For this reason, it is important that shaft and bearing materials are carefully selected so that they are compatible and do not undergo excessive wear.
Early pumps used in heating systems utilized shafts made of hardened stainless steel and bearings made of bronze (either solid or sintered). This combination worked well as long as the water (and thus the bearing lubricant) was clean. However, many heating systems suffer from galvanic corrosion due to the presence of copper (in pipes) and iron (in the boiler). The galvanic corrosion product is magnetite (chemical formula Fe3O4) which is very hard and abrasive. It precipitates out in the form of very fine particles which easily find their way into the bearing clearance where they have often caused severe wear of the stainless steel shaft. Even when hardened to the maximum possible extent, stainless steel is still softer than magnetite. To overcome this problem, shafts made of very hard ceramics such as alumina ceramic (chemical formula Al2O3) have been used. Unfortunately, the combinations of a bronze bearing and a ceramic shaft resulted in so-called “galling,” i.e., the transfer of bronze from the bearing to the shaft, thus destroying the bearing. In the past the bronze of the bearing has been replaced by carbon-graphite (i.e. the bearing system then includes a ceramic shaft and carbon-graphite bearing). An example of the use of carbon-graphite at a bearing surface is found in U.S. Pat. No. 5,549,459. This arrangement provided an improved bearing system. However, carbon-graphite bearings are very expensive.
Accordingly, it is an object of the present invention to provide an improved bearing system that provides a more durable bearing surface, while at the same time providing a bearing system that is relatively inexpensive, particularly in comparison to carbon-graphite bearings.
Another object of the present invention is to provide an improved bearing that is particularly adapted for use in a canned motor pump.